fix(backup): remove unsupported --storage-class COLD for Timeweb S3

Timeweb S3 doesn't support per-object storage class via API parameter.
Cold storage is configured at bucket level in Timeweb control panel.
Also: make S3 upload failures explicit (exit 1) instead of silently ignored.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

roles/backup/templates/backup.sh.j2

@@ -117,10 +117,9 @@ AWS_ACCESS_KEY_ID="{{ s3_access_key }}" \
 AWS_SECRET_ACCESS_KEY="{{ s3_secret_key }}" \
   aws s3 cp "${ARCHIVE}" "s3://{{ s3_bucket }}/data/$(basename "${ARCHIVE}")" \
     --endpoint-url "{{ s3_endpoint }}" \
-    --storage-class COLD \
     --no-progress \
   && log "  → S3 upload complete" \
-  || log "  ⚠ S3 upload failed (local backup still intact)"
+  || { log "  ⚠ S3 upload FAILED"; exit 1; }
 
 # ── Cleanup old local backups ─────────────────────────────────────────────────
 log "Removing local backups older than ${KEEP_DAYS} days..."